Cerebrospinal fluid (CSF) 

Cerebrospinal fluid (CSF) is a clear, plasma-like liquid that surrounds the brain and spinal cord. Produced by the choroid plexus, a network of cells in communicating cavities of the brain called ventricles, CSF primarily serves as a cushion between the brain and the skull that absorbs shocks and protects the central nervous system. In addition, CSF also circulates the important nutrients and chemicals that are extracted from the blood and helps in the removal of waste from the brain. Typically, the brain produces about five hundred milliliters of CSF per day. Doctors often take samples of CSF when they are attempting to diagnose nervous system diseases such as multiple sclerosis. Although the existence of fluid in the brain has been known since it was first described by the ancient Greek physician Hippocrates, it was not until many centuries later that scientists managed to develop an accurate understanding of CSF and what it does.

Background

Historically, the earliest clear reference to CSF came from fourth century B.C.E. Greek physician Hippocrates. In his writings on epilepsy, Hippocrates noted that brains of epileptic patients were flooded with phlegm, a substance that partially melted into water that subsequently surrounded the outside of the brain. Rather than thinking of this fluid as a naturally occurring substance, however, Hippocrates believed that it was the result of disease.

The next major step forward in the development of humankind's understanding of CSF came thanks to Galen of Pergamum, a Greek physician who studied medicine in the second century C.E. Galen, who gained much of his insight dissecting pigs and other animals because human dissection was banned under Roman law, closely explored the anatomy of the brain and described the ventricles. Critically, he suggested that the ventricles were filled with pneuma, a gaseous substance. Historians believe that this misconception arose from the fact that early scientists only examined the brain after the head was already removed from the body, allowing the CSF to leak out before it could be properly observed.

After the fall of the Roman Empire, European interest in science waned. As a result, little advancement concerning the study of CSF was made until the Renaissance. Using wax casts of the brain, famed Renaissance painter and scientist Leonardo da Vinci completed a remarkably accurate drawing of the ventricles and their anatomic structure. Andreas Vesalius, another Renaissance scientist, also studied the brain and suggested that while they normally contained Galen's gaseous pneuma, the ventricles were sometimes filled with a "watery humor."

The idea that the ventricles were filled with fluid rather than pneuma initially emerged in the eighteenth century. The first reasonably accurate description of CSF came from Domenico Cotugno, an Italian physician who dissected the skull while leaving the membranes that enclose the brain and spinal cord intact. This ensured that the CSF would not drain out and prevented air bubbles from forming inside the brain. By taking this approach, Cotugno was able to determine that the ventricles were indeed filled with CSF and not pneuma. In the decades that followed, scientists including Emanuel Swedenborg, who is typically credited with the formal discovery of CSF, continued to study and further refine the modern understanding of CSF and its functions.

Overview

The production and transportation of CSF is directly tied to the brain's ventricles. These ventricles are lined by ependymal cells that form the choroid plexus, a network of capillaries and connective tissue in which CSF is produced. There are four ventricles in total. Two of these, the left and right lateral ventricles, are located on either side of the cerebrum, or the main part of the brain. The third ventricle lies between the lateral ventricles and is connected to both. The fourth ventricle, which lies within the brain stem, connects with the third ventricle through the cerebral aqueduct. As CSF is produced, it flows from the lateral ventricles, through the third ventricle, and into the fourth ventricle. Once in the fourth ventricle, CSF drains into the subarachnoid cisterns (which bathe the brain) and the central spinal cord (which bathes the spinal cord). Eventually, CSF is absorbed back into the bloodstream and carried to the kidneys and liver, where it is filtered and excreted with other waste materials.

CSF has three main functions. First, it acts as a cushion that protects the brain from physical damage in the event of cranial injuries. In circulating key nutrients and chemicals and aiding in the removal of waste, CSF helps to ensure an ideal environment for proper functioning of the brain. Finally, CSF also slightly lowers the brain's weight, which prevents the base of the brain from being subjected to excessive pressure.

Normally, the production of CSF is a very precise process, with rate at which it is produced and absorbed being carefully balanced. When this balance is disrupted for one reason or another, the result is a condition called hydrocephalus. There are two main types of hydrocephalus: communicating hydrocephalus and non-communicating hydrocephalus. Communicating hydrocephalus is any abnormal collection of CSF not caused by an obstruction that disrupts the fluid's flow through the ventricles. Non-communicating hydrocephalus is any abnormal collection of CSF caused by an obstruction such as tumors, scar tissue, or cysts. Regardless of type, hydrocephalus often leads to headaches, vomiting, nausea, blurred vision, balance problems, bladder control problems, and thinking and memory problems. Treatment often requires the surgical placement of a shunt to improve CSF flow.

In some circumstances, CSF can actually leak out of the skull through the nasal passages. This condition, known as CSF rhinorrhea, occurs when there is a small hole or tear in the membrane that surrounds the brain. Often, such openings are the result of surgical complications or elevated pressure in the skull. In many cases, leaking CSF rhinorrhea is initially mistaken for an ordinary runny nose. While CSF rhinorrhea may resolve on its own, it often requires surgical intervention to correct. If left untreated, CSF rhinorrhea can lead to serious infections.

Doctors frequently rely on CSF to diagnose nervous system diseases such as multiple sclerosis. Samples of CSF are acquired through a procedure called lumbar puncture, or spinal tap. During this procedure, a long, thin, hollow needle is inserted between two of the patient's lower vertebrae and into the space where CSF is found. CSF is then extracted from the spinal cord. Tests performed on the patient's CSF can detect abnormalities and yield important diagnostic indicators of disease.

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